1. Field of the Invention
The present invention generally relates to an apparatus and method useful for determining the quality of the cement bond between a wall of a borehole penetrating an earth formation and a casing cemented therein. The quality of the bond behind one or more arcs of known location on the casing is determined by the present invention. The present invention further comprises an improved method of steering or directing the acoustic signal to maximize the signal transmitted and/or received. The present invention comprises an apparatus and method employing a plurality of sonic transducers in defined segmented locations for making the desired measurements.
2. Description of the Background
In the oil and gas industry, wells are completed by setting a string of pipe or casing in the borehole and filling the annulus between the pipe or casing and the borehole with cement. This cementing operation separates the various formation zones, and particularly separates the productive oil and gas bearing formations from non-productive formations such as water-bearing formations. Once separated by the cementing operation, only the desired oil and gas bearing formations are perforated for production. Failure or incompleteness of the cement bond likely will result in incomplete separation between the various formations. Migration of fluids under pressure through voids or cracks appearing in the cement between the casing and borehole wall causes contamination of the fluids of one zone with the fluids of another zone. This contamination is particularly undesirable where fluids from water-bearing strata migrate into fluids in a producing zone. This migration typically results in decreased production of hydrocarbon fluids and increased production of non-desirable fluids and contamination of equipment. These results often cause serious financial loss and may even cause a well to become non-commercial.
Accordingly, the desirability of accurately determining the quality of the cement bond between a casing and a wall of a borehole has long been a goal of those associated with the oil and gas industry. A "good bond" produces the desired separation of zones and is achieved with good adhesion, although micro-fissures or a micro-annulus may be present.
Many prior systems have been developed for investigating the bond quality between a casing and a borehole wall. Prior systems for inspecting the cement bond in wells in the oil and gas industry have included conventional acoustic cement bond systems employing acoustical energy in a variety of devices and methods. In fact, such a system is described and claimed in U.S. Pat. No. 4,255,798 to Havira which employs an acoustic pulse echo technique for investigating the quality of the cement bond to a casing and the thickness of a casing located in a borehole. Further, the Havira patent lists and describes in columns 2-7 a plurality of patents and articles describing in detail many prior art systems for evaluating cement bond quality using acoustic energy. The Havira patent and its extensive description of these prior systems are incorporated herein by reference.
However, these prior acoustic systems typically suffer from one or two major drawbacks. Many of these prior systems merely provide an average indication of the bond quality about the entire circumference of the casing at the measured depth within the borehole. Accordingly, the investigator using such systems cannot determine the specific azimuthal location about the borehole where a channel may be located. In fact, the investigator cannot distinguish a troublesome channel from several small fissures. Other systems which do provide azimuthal scanning measure the bond condition at only specific points around the circumference and give no indication of the bond condition over the arc between those points.
Accordingly, there has been a long felt but unfulfilled need within the oil and gas industry for an effective apparatus and method useful for providing an inspection system for determining bond quality in specific, azimuthally located arcs of a cemented casing.